1. Field of the Invention
The invention relates to aircraft navigation and guidance systems and, more particularly, to a system for rapidly determining target speeds for meeting required times of arrival while correspondingly maintaining optimum fuel efficiency.
2. Description of Related Art
Over the past decades, the cost of petroleum fuels and the cost of labor have greatly increased, affecting many industries, including the air transportation industry. With respect to jet aircraft, the amount of fuel consumed in relatively long distance flights will be several thousands of pounds. Typically, fuel consumption can be optimized by flying aircraft at certain speeds and altitudes, subject to aircraft operation constraints, such as engine pressure ratio (EPR) limits of the aircraft engines.
Historically, booklets comprising charts having various engine pressure ratios, altitudes, speeds and the like have been compiled for use by the pilots. Prior to flight, the pilots would review the chart materials and select a particular flight profile, while maintaining aircraft performance. However, discrepancies between the real parameters of the aircraft and the theoretical parameters utilized to obtain the charts resulted in substantial errors, further resulting in excess fuel burned and excess time required in flight.
During the past two decades, numerous systems have been developed for on-board aircraft use for utilizing essentially "real time" aircraft variables for determining optimum speed/altitude flight profiles. For example, one such device is disclosed in U.S. Pat. No. 3,153,143 issued to Fogarty on Oct. 13, 1964. The Fogarty device utilizes real parameters, including weight and the change of weight as fuel is consumed, to compute the maximum range or maximum endurance. Another such device is disclosed in U.S. Pat. No. 3,613,837 issued to Brandeau on Oct. 12, 1971. The Brandeau device includes a warning indicator actuated when excessive fuel consumption is detected. Brandeau employs real data of the flight to compute fuel reserves, time to target, and the range or endurance of the flight at different altitudes.
Although fuel consumption is of primary importance with respect to economical flight, other cost factors are also of importance. For example, the crew of an aircraft can involve many people, both on the ground and in the air. The wages of such crews are often dependent on the flight time duration. Such time-dependent costs may be relatively lower at greater speeds (in view of the shorter flight durations), but fuel costs may be correspondingly increased.
A significant advance in the state of the art for flight performance systems was achieved with the performance data computer system disclosed in U.S. Pat. No. 4,312,041 issued to DeJonge on Jan. 19, 1982. The DeJonge patent discloses a flight performance data system having a control and display unit operably connected to a computer. The computer is responsive to input variables comprising real time measurements, such as air mass parameters and aircraft dynamic measurements, for automatically generating various output signals representing flight profile data.
In the system, a mode selector is operably connected to the computer in different modes to determine flight profile data for modes such as climb, cruise, and descent. For the various modes, different flight schedules can be determined corresponding to the most fuel-efficient flight, a long-range cruise flight, or the most economical flight. In part, the DeJonge patent discloses the use of a cost index factor representing a ratio of time-dependent costs for the flight, relative to flight fuel costs. The cost index factor is utilized to provide signals to the pilot representing optimum flight profile data on an economic basis.
Numerous other systems have also been developed for use in aircraft flight control. These systems include various arrangements for generating flight profile data, and also for automatically controlling the aircraft throttle and other aircraft instrumentation. These systems include the generation of output data representing estimated times of arrival (ETA's) to destinations, including not only final destinations, but also "waypoints," representing benchmark navigational coordinates of locations along a flight path.
In addition to problems associated with development of better procedures and methods for estimating and maintaining optimum flight plans within aircraft and atmospheric constraints, the airline industries have been coping with significant increases in air traffic. Historically, air traffic control (ATC) has been performed in part by local airport traffic controllers for the regions surrounding the airport. That is, as an aircraft approached the local destination airport, the local controllers would essentially "take over" control and direct the pilot to maintain certain positions and speeds during descent.
During flight between the local airport centers, air traffic control has essentially been performed by regional federal air route traffic control centers. Such control has typically involved the issuance of instructions by the federal controllers to the pilots in the form of speed, altitude, and flight path directional parameters for purposes of maintaining at least minimum spacing between aircraft. In addition to flight path control, aircraft have typically been assigned certain take-off and destination arrival times. These designated times have been coordinated through a federally-controlled computer system.
Notwithstanding the sophisticated and complex systems employed by federal and local airport authorities for directing aircraft around airports and en route to airport destinations, these systems have been "stretched" to capacity in view of current air traffic. More specifically, these systems essentially lack an overall coordination to optimize air traffic flow and aircraft performance. For example, when a local airport control area begins to reach arrival capacity, the local control authorities have had the capability of controlling traffic only within the local area. An aircraft en route to the local airport may be operating at a relatively high (and fuel inefficient) speed to meet a designated gate arrival time, only to find that the aircraft is "stacked up" and further delayed when it reaches local air space. This further delay serves to increase fuel use, in addition to adding local traffic at an airport which may already be operating above capacity. Further, stacking of aircraft clearly includes other disadvantages, such as potentially reduced safety and increased aircraft noise for local communities.
Several years ago, the Federal Aviation Administration (FAA) issued directives for purposes of establishing a "local flow traffic management program" designed to enhance airport safety, reduce impact of aircraft noise on local communities, and conserve aviation fuel. These directives specifically related to review and revision of procedures utilized by air traffic divisions and air route traffic control centers to reduce flying times at low altitudes (e.g. below 10,000 feet) and provide for maximum use of profile descents from cruising altitudes to an approach gate.
The directives issued by the FAA also established a "metering" program for purposes of developing procedures to monitor the arrival flow, so as to determine when the number of aircraft approaches system capacity. This concept, often referred to as "en route metering" (ERM), essentially allows aircraft to absorb any necessary arrival delays en route, with transition to the terminal area in a pre-planned sequence based on a calculated airport acceptance rate. When delays would be imposed, the priority of landing would be based on the calculated time of arrival (CTA) for each aircraft. These calculated times of arrival would be based on the estimated time of arrival at a meter fix, plus the estimated flying time to the runway. These times would then be adjusted to resolve simultaneous demands at the airport, and to establish a time that an arriving aircraft would be required to cross a meter fix. With these procedures, several advantages are intended. For example, a primary concept is to minimize congestion and delay in airport terminal air space, thereby reducing noise around the major airports and also reducing the exposure of high performance aircraft to low performance aircraft. In addition, it is also desirable to prevent unnecessary delay of arrival flights, by removing or reducing current flow restrictions. Still further, it is a primary advantage to essentially move the unavoidable delay outside of the termination air space. In this manner, aircraft delays would be essentially more fuel efficient, since the delays would occur during high altitude flight. In addition, for relatively short range flights, delay can occur while the aircraft is maintained on the ground prior to take-off. In general, a primary objective of en route metering is to organize arrival traffic in en route air space, by providing interfacility coordination of arrival delays and also by assisting in the merging of arrival flights.
Early operational experience with the FAA directives essentially indicated that the principles associated with the directive and the requisite procedures would be effective. However, metering functions previously had to be accomplished manually, requiring a significant amount of resources in preparing metering strips, computing times and requisite updates. Accordingly, subsequent governmental directives were issued for providing guidance and procedures for the development, implementation and operational utilization of an automated ERM program. During the early part of the 1980's, ERM procedures were in place at a number of major airports.
In further explanation of the ERM concept, the primary objective is to allow aircraft to absorb arrival delays, based upon current "airport acceptance rates", during the en route phase of flight under substantially more fuel-efficient conditions. Further, the aircraft would then be "fed" into the airport in a pre-planned, orderly sequence. This type of arrival sequence is essentially a first come-first served sequence, and is based upon projected airport arrival times.
The ERM function is essentially dependent upon the local adaptation of several input meter parameters, and provides meter-related messages for use by control personnel in the metering process. The metering process is essentially a method of time regulating, where necessary, arrival traffic flow into a terminal area without exceeding the airport acceptance rate, and by allowing arrival delays to be absorbed at higher altitudes en route. To efficiently and accurately apply this concept, arrival traffic must be consistently routed into a terminal area via designated metering routes (standardized flight paths) primarily consisting of an outer fix, meter fix, and a predetermined route to an active runway (vertex).
With ERM procedures, each metered arrival is assigned a meter fix time based on the airport acceptance rate and the aircraft's CTA at the vertex. Conflicts between two or more aircraft determined to reach the vertex at approximately equal times are accounted for when runway times are actually assigned. By establishing a priority sequence for arrivals, based upon their scheduled landing times, and computing their meter fix times accordingly, "first come-first served" is effectively achieved with the distribution of arrival delays in an "equitable" manner among all metered aircraft.
As part of the en route arrival metering strategy, a primary concept is to predict the time each aircraft will arrive at the runway, if no control is exercised. With the previously-established metering procedures, this strategy is accomplished dynamically, while the aircraft are typically at en route altitudes and in a range, for example, of 150 to 175 nautical miles away from the runway. Based on this initial arrival time estimate, a desired runway (or airport) schedule is created which provides for interoperation times consistent with airport capacity, and which resolve conflicting runway use. Accordingly, a desired time slot is created for each aircraft, perhaps 30 minutes prior to when the actual operation is to occur. This "list" of assigned runway times is then adjusted by the transition times from meter fixes to the runway, so as to achieve an assigned time at a meter fix. This assigned meter fix time is utilized by the en route sector controller to "deliver" each aircraft to the traffic controller. The traffic controller will utilize speed control and/or vectoring so as to achieve delivery within an accuracy of .+-.one minute. Adequate separation must be maintained between the aircraft at all times. In addition, with flow rate essentially being the primary parameter for the metering strategy, a controller may actually interchange or "swap" times assigned to two aircraft en route.
Air route traffic control centers have instituted control positions characterized as the arrival sequence controllers. These positions are involved in the sequencing and scheduling of airplanes, rather than the actual control which is eventually accomplished by the sector controller providing the meter fix. The arrival sequence controller essentially operates with computer-generated "flight strips" indicating aircraft identification, type, arrival meter fix and estimated arrival time at the meter fix. In an original implementation, the estimated meter fix time was generated by projecting the airplane from its current position to a meter fix based on flight plan speed. A nominal transition time value was then added to the meter fix estimate, so as to obtain the estimated landing time. The TMA transition times were typically based on a table of values, giving the time as a function of the particular meter fix and aircraft-type category. Also, meter fix time estimate corrections could be provided to the arrival sequence controller by the system, based on various changing parameters.
Various software packages have been developed for providing geometry/procedures/traffic data specific to given terminal areas. Approach times are calculated based on the specification of a sequence of approach "legs." Such an approach leg is described in terms of path distance, heading, altitude and true air speed (which may also depend on aircraft type). Impact of wind on flight leg transition times can also be factored into the calculations. In this manner, each arrival is essentially assigned a "nominal" approach path, comprising a sequence of approach legs for computing the estimated TMA transition time.
With this data, the arrival sequence controller reviews the list of landing times so as to determine, for example, whether the airport capacity will be exceeded for any ten minute period. If so, the sequence controller will also determine if any landing conflicts may occur. If a ten minute demand will be exceeded, the controller can progress from "passive metering" to "active metering." The metering program will determine successive capacity time slots for each aircraft, based on the sequence established by the initial landing time estimate. These assigned landing times are then re-adjusted for nominal TMA transition times, so as to obtain an assigned time at the meter fix. This assigned meter fix time can then be displayed to the appropriate sector controller, with the sector controller then controlling the aircraft so as to maintain the assigned times. With this arrangement, only aircraft for which a landing time exists will be allowed into the "near-terminal" region. In addition, this process also essentially equitably distributes air traffic control delay, and provides visibility as to when and how much holding may be required in an en route area. The automation procedures essentially provide various algorithms so as to perform runway time calculation, runway scheduling and meter fix time assignment functions, with the results displayed at the arrival sequence controller position, and with results displayed to the appropriate sector controllers when active metering is in progress.
For purposes of achieving a nationwide en route metering strategy, it is advantageous for aircraft to have the capability of dynamically adjusting speed and/or flight paths in response to a designated metering fix, for purposes of crossing the meter fix at the designated time (within certain precision tolerances). This procedure is often characterized as time controlled navigation (TNAV) and guidance. Time-controlled navigation and guidance is the ability to specify a desired crossing time at a selected waypoint, and provide control signals to enable an aircraft to meet a crossing time objective. As previously described, aircraft controllers can utilize specified crossing times in en route metering, along with radar monitoring, so as to establish orderly traffic separation for aircraft arriving into a limited capacity local control area. This concept of utilizing commands in a time-controlled navigation system is also characterized as "4-D control."
4-D control can be achieved by various procedures. For example, the aircraft can execute holding patterns, or other path stretching maneuvers, when required to delay the arrival time. Also, aircraft speed adjustments can be made as required. Commands for providing this dynamic adjustment of aircraft speed and/or flight paths can be received by the aircraft from different sources. For example, commands related to speed and flight paths can be directly received from air traffic control computer systems. Also, such specific commands can be received directly from controllers. Still further, and of primary importance, it is now known to utilize on-board aircraft computer systems for purposes of providing command signals to the aircraft and for generating flight profile data based on various parameters. In particular, such on-board computer systems have been utilized for generating relatively more fuel efficient flight profiles, thereby reducing fuel consumption and providing cost savings. These known on-board systems include the capabilities for performance of strategic and tactical flight planning, providing aircraft performance calculations, displaying relatively precise navigation and guidance data, and further providing the capability of direct interfacing with airline and air traffic control data-links. The basis for many of the known on-board computer systems is the capability of providing, in real time, optimum speed commands and altitude data. Since these on-board computer systems, often referred to as flight management systems (FMS) or flight management computer systems (FMCS) receive real time data from sensor inputs, typically include data bases comprising aircraft performance characteristics, and navigation facility and routing information and have the capability of being coupled to autopilots/autothrottles, such systems are particularly advantageous for performance of 4-D control.
The principal elements of a time-controlled navigation or 4-D control capability include the following: (1) a performance definition section, generating a four-dimensional reference path based on airplane performance and navigation data; (2) a navigation section, in which sensor data are used to calculate a current aircraft state vector (i.e. location, direction and velocity); (3) the guidance section, in which the current state vector is compared to a reference path and differences (error signals) determined in a form suitable for driving the flight control system; and (4) a flight control system which commands the aircraft aerodynamic controls and engine thrust, as necessary, to respond to guidance signals and actually "fly" the desired 4-D control path. A time-controlled navigation procedure can provide substantial benefits over basic methods of control, in view of increased precision for meter fix arrival times, relatively more fuel-efficient flight, reduced pilot workload, and reduced controller workload.
Until recently, use of time-controlled navigation and specifications of desired crossing times have been limited to utilization within traffic control centers, since aircraft systems have not had the capability of making substantial use of this data. To provide for optimum use of desired crossing time information, the following elements are required within aircraft systems: (1) accurate lateral and vertical navigation, with waypoint naming conventions consistent with air traffic control procedures; (2) the ability to accurately predict earliest and latest arrival times at a designated waypoint within an aircraft's performance envelope, in order to provide an indication as to the capability of compliance with an air traffic control request; (3) active speed control within an aircraft's performance envelope, for purposes of meeting the time objective; (4) an indication alert when an aircraft can no longer meet a time objective within a given tolerance; and (5) guidance for lateral delay absorption maneuvers, for purposes of meeting the time objective.
Currently, flight management computer systems are available which have the capability of accurately predicting flight profiles, while correspondingly taking into account lateral and vertical flight plans, aircraft limits, and predicted atmospheric conditions. For purposes of providing time-controlled navigation, the capability of predicting crossing times with relatively high accuracy is of primary importance. In known flight management computer systems, this accuracy is dependent in part on the accuracy of predicted wind conditions.
As previously described, a parameter of primary importance in aircraft flight management computer systems is the cost associated with the flight. As will be described in subsequent paragraphs herein, this parameter is typically referred to as a "cost index" (CI), utilized to describe relative costs between flight time and fuel consumption. An advantageous feature of utilizing the cost index for time-controlled navigation is that the cost index affects climb, cruise and descent profiles in a manner so that profiles can be computed which result in minimum fuel consumption for the corresponding flight times.
In view of the foregoing, assuming that the aircraft is provided with waypoints and required times of arrival by the air traffic controller (or otherwise), it would appear ideal to enter the waypoint and RTA data into the flight management computer system and then await generation of new flight profile data. In general, the flight management computer system would be programmed to essentially "search" for the optimum cost index, while still meeting the time constraints of the designated required times of arrival. However, with on-board processors currently implementing flight management computer systems, flight profile prediction computations may require a substantial period of time to complete. Correspondingly, with search techniques such as a conventional binary search algorithm for purposes of searching for the cost index required to meet a particular time constraint, a number of trial "passes" at flight profile predictions would be required. If the system is being utilized for purposes of achieving active control of the aircraft, such long computation and search times are essentially unacceptable.